Self-similar laser oscillator

ABSTRACT

A laser producing high energy ultrashort laser pulses comprises a normal dispersion segment, a gain segment, an anomalous dispersion segment with negligible nonlinearity and an effective saturable absorber arranged to form a laser cavity. Each segment is optically interconnected so that a laser pulse will propagate self-similarly therein. (A pulse that propagates in a self-similar manner is sometimes referred to as a “similariton.”) With this laser the limitations of prior art laser oscillators are avoided. Also provided are means for pumping the gain medium in the laser cavity, and means for extracting laser pulses from the laser cavity. The laser cavity is preferably a ring cavity. Preferably the laser is configured to achieve unidirectional circulation of laser pulses therein. This configuration is scalable to much higher pulse energy than lasers based on soliton-like pulse shaping.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract (entercontract numbers). The government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lasers, and more particularly to fiber lasers.

2. Description of Related Art

Lasers, particularly those partially or wholly incorporating opticalfibers, have emerged as attractive sources of extremely short pulses oflight. Fibers lasers are of particular interest because they can betightly coiled to produce long path lengths in compact geometries. Andbecause fiber lasers can be hard-wired, they can be made impervious toadverse environmental effects—especially when the polarization is fixedin a manner that makes them relatively insensitive to mechanicallyand/or thermally induced birefringence effects. They can also be arelatively inexpensive, cost-effective solution to the generation ofshort pulses of light. Rare earth-doped fibers, and in particularneodymium-doped or ytterbium-doped fibers, are particularly advantageousfor fiber laser designs because they can be diode pumped and arescalable to high powers. Mode-locked fiber lasers incorporating rareearth-doped fibers have been shown for in several configurationsincluding linear, ring, and figure-eight geometries.

The energy of the pulses generated in fiber laser oscillators isgenerally limited by effects that cause the pulse to break up intoseveral pulses (called wave-breaking.) Wave-breaking is a consequence ofexcessive nonlinearity within the oscillator cavity—a limitation that isparticularly problematic in ultrashort pulse fiber lasers where thesmall beam diameter produces high intensities and therefore largenonlinear phase shifts. Prior art work, while descriptive of thesepropagation effects in fiber amplifiers did not anticipate this regimeof operation in a laser oscillator. In fact, the operating regimedescribed in this invention, which corresponds to an asymptotic solutionof the governing wave equation, would appear to be incompatible with theperiodic boundary condition required for stable operation of a laseroscillator. This is novel, and has not been anticipated in prior artlaser oscillators.

This invention describes a laser oscillator that tolerates strongnonlinearity without wave-breaking. This text teaches both the numericalpredictions and experimental evidence of the existence of a stable pulsethat propagates “self-similarly” within a laser oscillator cavity (asimilariton pulse.) By propagating self-similarly we mean that, whilethe amplitude and phase of the pulse may vary as it propagates aroundthe cavity, the shape of the pulse does not. This regime of operationconstitutes a new type of pulse shaping in a modelocked laser,qualitatively distinct from the well-known soliton (see Kafka and Baer,Optics Letters 14, 1269 (1989) and U.S. Pat. No. 4,835,778 bothincorporated herein by reference) and stretched-pulse laser (see forexample U.S. Pat. No. 5,617,434 incorporated herein by reference in itsentirety) regimes (stretched-pulse lasers are referred to herein asdispersion-managed (DM) soliton lasers.)

More particularly, those familiar with the art will recognize that pulseformation in lasers that produce pulses of short duration is dominatedby the interplay between dispersion and nonlinearity, in the form ofsoliton dynamics. These pulses are not exact solitons, because the lasercavity constitutes a dissipative system. Hence, their basic features canbe understood within the formalism of a complex Ginzburg-Landauequation. Soliton-like dynamics are particularly strong in fiber lasers;the cavity length typically corresponds to several times the dispersionlength, which is the propagation distance over which a pulse wouldbroaden by approximately a factor of 1.4 owing to linear group-velocitydispersion. An effective saturable absorber (SA) is required forinitiation of pulsed operation from intra-cavity noise and subsequentstabilization of the pulse.

Soliton fiber lasers are limited to low pulse energies (100 pJ or less).At higher energies, nonlinear effects cause wave-breaking which leads toa transition to multiple-pulsing (more than one pulse circulating withinthe cavity). The pulse can tolerate only a small nonlinear phase shift(substantially less than one radian) within one round-trip beforeinstabilities occur. Stretched-pulse fiber lasers of the type describedin U.S. Pat. No. 5,617,434 consist of segments of anomalous(soliton-supporting) and normal (non-soliton-supporting) GVD. Theseimplement the concept of dispersion management (DM), and support theanalog of DM solitons. DM solitons can tolerate nonlinear phase shiftsan order of magnitude larger than ordinary solitons, and the pulseenergy thus exceeds the soliton energy by the same factor.

Solitons are static solutions of a nonlinear wave equation, and DMsolitons are breathing solutions, both of which satisfy the periodicboundary conditions with feedback required for stable oscillatorybehavior. In contrast, pulses that propagate self-similarly areasymptotic solutions to the governing equation. The evolution of theirproperties (e.g., the pulse duration) is monotonic. Such a solutioncannot satisfy periodic boundary conditions. An additional mechanism isrequired to obtain a stable operation in a cavity. Also pulses in alaser will, in general, evolve to fill available gain bandwidth.However, self-similar propagation of intense pulses is disrupted if thepulse encounters a limitation to its spectral bandwidth.

BRIEF SUMMARY OF THE INVENTION

A model fiber laser cavity includes a segment of single-mode fiber (SMF)with normal dispersion that forms a large portion of the cavity length.The pulse propagating in the cavity has a central wavelength that lieswithin the gain profile of the gain medium. As the pulse propagatesthrough the SMF fiber, its bandwidth is broadened through nonlinearinteraction with the material. This effect operates in combination withgroup velocity dispersion (GVD) to create an approximately linear chirpon the pulse. Amplification is provided by a segment with gainsufficient to achieve the desired pulse energy. The length of this gainsegment is minimized so that the pulse will experience negligible GVDand nonlinearity during amplification—effectively decoupling bandwidthfiltering from nonlinear evolution in the fiber. The gain fiber isfollowed by an effective saturable absorber (SA), which may also serveas an output coupler. The final element is a dispersive delay line (DDL)that provides anomalous dispersion with negligible nonlinearity. Chirpaccumulated in self-similar propagation will be compensated by the DDL,while any excess bandwidth will be filtered by the gain medium. Thisconfiguration produces a self-consistent solution in the resonatorcavity and results in stable operation of the laser. The evolution ofthe pulse as it circulates one round trip in the cavity is illustratedby the plot of frequency chirp versus cavity position, shown in FIG. 1A.

Numerical simulations of the model laser were employed to search forself-similar solutions to the governing equations and to verify thatsuch solutions are global attractors (that is, self-starting isaccessible from intracavity noise.) The parameters of the numericalmodel correspond to the preferred embodiment. A 4 meter long segment ofSMF is connected to a 20 centimeter long gain fiber, and a grating pairis used for the DDL. Propagation within each section is modeled with anextended nonlinear Schrodinger equation

where $\xi$ is the propagation coordinate, $\tau$ is time scaled to thepulse duration, $\beta_(—){2}=23 fs{circumflex over ( )}2/mm$ is theGVD, and $\gamma=0.0047 (Wm){circumflex over ( )}{−1}$ is the effectivecoefficient of cubic nonlinearity for the fiber section. $\gamma$ is setto zero in the DDL, which is adjusted to achieve a desired totaldispersion of the cavity. The pulse energy is given by$E_{pulse}=\int{circumflex over( )}{T_{R}/2}_{−T_{R}/2}{|A(\xi,\tau)|{circumflex over ( )}2 d\tau}$ and$T_{R} \sim 30$ ns is the cavity roundtrip time. $g(E_{pulse})$ is thenet gain which is non-zero only for the amplifier fiber. The gainsaturates with total energy according to

\begin{equation}

g(E_{pulse})=\frac{g_(—){0,\omega}}{1+E_{pulse}/E_{sat}},

\end{equation}

where $g_(—){0,\omega} \sim 30$ dB is the small signal gain with aparabolic frequency dependence, and a bandwidth of $\sim 40$ nm isassumed. The effective gain saturation energy, $E_{sat}$, is set to 0.5nJ. The SA is assumed to saturate monotonically, and is modeled by atransfer function that describes its transmittance

\begin{equation}

T=1−\frac{1_(—){0}}{1+P(\tau)/P_{sat}},

\end{equation}

where $T$ is the transmittance, $I_(—){0}=0.2$ is the unsaturated loss,$P(\tau)$ is the instantaneous pulse power and $P_{sat}=2000$ W is thesaturation power. The pulse amplitude is scaled down by a factor of$\sim 10$ after the SA to account for other linear losses in theexperimental setup as described below.

The simulations exhibit stable pulse formation over a reasonably-widerange of parameters. Several aspects of a typical solution are shown inFIG. 1B. The approximately-parabolic temporal profile, nearly linearchirp, characteristic shape of the power spectrum, and the variation ofthe intensity profile are all signatures of self-similar pulseformation. For reference, the results are compared to a laser thatsupports DM solitons. Self-similar pulse propagation does not increasethe maximum stable pulse energy when the net group velocity dispersionis anomalous (${\rm \beta_{net}} \|approxeq 0$). However, for increasing${\rm \beta_{net}}>0$ (normal dispersion) the pulse energy divergesdramatically (see FIG. 1C) from that expected for a DM soliton laser.The stretching ratio for the laser with self-similar evolution of thepulse decreases exponentially at fixed pulse energy as the net groupvelocity dispersion is increased. Thus, the energy-scaling can beunderstood intuitively as follows: the (exponential) increase in pulseenergy with increasing GVD maintains a fixed stretching ratio.

The fiber laser of the invention overcomes the limitations of past fiberlasers to provide a laser operating in the self-similar regime. Thisregime is distinguished from the previously known soliton and stretchedpulse regimes in that it is more tolerant of nonlinear effects inducedon the pulse by propagation through the fibers with normal groupvelocity dispersion. Thus this self-similar laser operates outside ofthe soliton and stretched pulse fiber laser regime and is capable ofproducing much higher energy ultrashort laser pulses.

The invention thus provides, in one general aspect, a fiber laser havinga long, undoped, net normal group velocity dispersion single mode fibersegment joined in series with a rare earth doped, net normal groupvelocity dispersion fiber segment which is in turn joined with aneffective saturable absorber followed by a linear net anomalous groupvelocity dispersion segment. With this configuration, self-similarpropagation of a laser pulse in the cavity is made possible in a mannerthat satisfies the periodic boundary conditions required for stablelaser operation. The fiber laser also provides means for modelockinglaser radiation in the laser cavity, means for pumping the gain mediumin the laser cavity, and means for extracting laser pulses from thelaser cavity.

In a preferred embodiment, the laser cavity exhibits a net positivegroup velocity dispersion.

In another preferred embodiment the gain in the laser cavity comprises asegment of rare-earth doped material, such as neodymium- or ytterbium-or erbium- or thulium-doped material and may also include more than onedopant (known as a co-dopant) for purposes such as facilitating energytransfer from a specific pump wavelength to the absorption wavelength ofthe gain material. The rare-earth doped material preferably possessesnet positive group velocity dispersion.

In other embodiments, the laser cavity comprises a linear cavitygeometry, a ring cavity geometry, or a figure eight geometry, as isknown to those skilled in the art.

In yet another embodiment the cavity is configured to achieveunidirectional circulation of laser pulse in the cavity.

In yet another embodiment the laser pulses extracted from the ringcavity preferably exhibit a pulse energy of at least 1 nJ.

In another embodiment, the modelocking means provides passivemodelocking of the laser radiation in the laser cavity via nonlinearpolarization evolution (NPE). A nonlinear polarization rotator useselliptically polarized laser pulses to provide intensity dependent lossand transmission of laser energy such that the substantially off-peaklaser pulse intensity energy is rejected and substantially on-peak laserpulse intensity energy is passed to continue circulating in the lasercavity.

In another embodiment the nonlinear polarization rotator comprisesall-fiber components, or bulk components partially or completelyinterconnected with fibers.

In another embodiment, the modelocking means is accomplished through theKerr nonlinearity via the Kerr lens effect.

In yet another embodiment the modelocking means is accomplished througha semiconductor material.

In yet another embodiment the gain media is chosen from a listcomprising, a bulk gain material, a semiconductor, and or gain fiber.

In other embodiments, one or more wavelength-division-multiplexed (WDM)input coupler(s) is used to couple the pump radiation into and out ofthe laser cavity.

In other embodiments a semiconductor diode light source, or array ofsemiconductor diode light sources, is used to pump the gain medium.

In another embodiment at least one diode-pumped solid state source isused to pump the gain medium within the laser cavity. In one embodimentthe means for extracting a portion of the intracavity laser pulse is aWDM coupler.

In another embodiment the means of extracting a portion of theintracavity laser pulse is via the NPE rejection port.

In yet another embodiment both an NPE rejection port and a WDM outputcoupler are incorporated in the same laser cavity to provide two outputsfrom the laser.

In yet another embodiment the output from the laser is coupled into anamplifier either directly or through a fiber stretcher for subsequentamplification, as is known to those skilled in the art.

In another preferred embodiment the anomalous dispersion segment withnegligible nonlinearity comprises one or more bulk optical elementschosen from a list comprising a prism or a grating.

In another preferred embodiment the anomalous dispersion segment withnegligible nonlinearity comprises a hollow microstructure fiber.

In yet another embodiment one or more fiber segments are chosen from alist comprising bulk optical elements, single mode fibers, multi-modefibers, microstructure fibers, photonic bandgap fibers, or large modearea fibers.

In one embodiment the laser pulse extracted from the laser cavitypropagates through a segment of hollow microstructure fiber to compressthe pulse.

In yet another embodiment, the extracted laser pulse propagates througha segment of hollow photonic bandgap fiber whose wall is doped with again element that is pumped to provide gain for the pulse propagatingtherein.

In another preferred embodiment an arrangement of gratings and/or prismsare employed to compress the pulses extracted from the laser.

The fiber laser of the invention achieves high-energy ultrashort laserpulses by intentionally operating outside of the dispersion-managedsoliton regime, contrary to conventional theories suggesting thatoperation in dispersion-managed soliton is not only preferable butrequired by the periodic boundary conditions required for stable laseroscillation with feedback. The inventors herein have recognized thatthis new regime of operation eliminates the limitations imposed by theprior art designs such as the soliton and stretched-pulse fiber lasers.They also recognized that this laser oscillator is capable of producingpulse energies substantially larger than those achieved in previousdesigns. In addition the inventors anticipate that this laser will haveextremely low noise—an operating characteristic that is very useful inapplications where it is important to operate close to threshold toachieve the desired outcome, or where a high degree of reproducibilityis important, as, for example, is the case in the technology describedin U.S. Pat. No. RE 37,585 and divisionals thereof incorporated hereinin their entirety by reference.

The fiber laser of the invention has a wide range of applications in thefields of research, instrumentation, biomedical engineering,spectroscopy, telecommunications, micromachining, ablation,microfabrication, multi-photon photopolymerization, multi-photonmicroscopy, and multiphoton imaging, multi-photon ionization, and x-raygeneration, to name but a few of the possible multi-photon initiatedapplications for which this laser is suited. The fiber laser describedherein could also be used as a seed source for amplifier systems thatare both bulk optical element and/or employ fibers, and that areoperated in one or more of a single pass, multi-pass, or chirped pulseamplifier schemes. The many advantages of this compact, inexpensive,robust source of pulses may thus be widely employed to advancetechnology and applications beyond their current level.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 2 is a schematic Illustration of a preferred embodiment of theinvention;

FIG. 3 is a schematic diagram illustrating how the pulse propagatesself-similarly within the preferred embodiment of the laser cavity;

FIG. 4 is a table comparing the invention with prior art soliton andstretched-pulse lasers;

FIG. 5 shows the characteristic near parabolic shape of the spectrumthat is typical of self-similar operation of the invention;

FIG. 6 shows the pulse from the preferred embodiment before dechirping(red curve) and after dechirping (yellow curve) using a pair ofgratings.

FIG. 7 shows the spectral shape of a version of a self-similar(similariton) laser designed to produce maximum peak power, i.e. highestpulse energy in the shortest possible pulse.

FIG. 8 shows the autocorrelation trace of the dechirped pulse from ahigh pulse energy self-similar laser oscillator.

FIG. 9 schematically illustrates the main features an all-fiber versionof the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, the fiber laser 10 of the invention is shown asa ring laser. The fiber laser of the invention could also be configuredas a figure-eight cavity, linear cavity or other suitable lasergeometry, as will be understood by those skilled in the art. The ringgeometry described herein is but one example of the many geometries towhich the invention could be applied.

The fiber ring laser of FIG. 2 comprises several segments. On the rightside of the ring, we see segment 20 of the ring laser 10 of theinvention comprises the majority of the optical length of the cavity andgenerally supports only a single spatial mode. In a preferred embodimentthis segment of fiber single mode fiber (SMF) has a length of 4 meters.Segment 20 is characterized by a net positive group velocity dispersion(also referred to as net normal dispersion). One end of segment 20 isconnected a collimator 22 to receive a pulse circulating in the cavity.The other end is connected to a wavelength division multiplexer (WDM)coupler 30 to couple the pump light from one or more laser diodes 40into the cavity via fiber 50, as shown (a WDM coupler is but one of manymeans known to those skilled in the art whereby pump light may becoupled into a fiber. Other examples include a V-groove coupler (ref.V-groove patent), a star coupler (Star coupler patent?). The means bywhich pump light is coupled to the gain medium is not critical to theinvention and is not intended to be limiting in any way.

Connected to the other end of WDM coupler 30 is a second segment offiber 60—also characterized by a net positive group velocity dispersion.Segment 60 is doped with a rare earth element. Segment 60 has a dopantconcentration and length chosen to minimize nonlinear pulse propagationeffects while maximizing gain therein. In a preferred embodiment, thelength is chosen to be 23 cm and the fiber is doped with Ytterbium to alevel of 23,600 ppm. The opposite end of fiber segment 60 is attached toa collimator 70 to collimate the light exiting the fiber. Opticallyconnected to the collimator is a quarter wave plate 90, a half waveplate 100, and a polarization beam splitter 110. Taken together, thesethree elements form the “nonlinear polarization evolution” (NPE) sectionof the cavity.

Optically connected to the NPE section is a net anomalous dispersionsection of the cavity. In the illustrative embodiment this sectioncomprises four gratings, 120, 130, 140, and 150 aligned and spaced sothat a chirped pulse entering this section from the left is compressedto a minimum value as it exits this section on the right. Preferablythis anomalous dispersion section of the cavity possesses negligiblenonlinearity in order to avoid soliton pulse shaping effects therein.This section is followed by an optical isolator 160, and a secondquarter wave plate 170 which serve to favor unidirectional propagationof light within the cavity and facility self-starting. All the aboveelements are optically connected in series and aligned so that a pulseof light circulates in the cavity.

FIG. 3 schematically illustrates how a pulse propagates self-similarly(i.e. without substantial variation in the shape) in this ring cavityconfiguration. Starting at point 200 just after the quarter wave plate170, the pulse enters into through fiber segment 20 its bandwidthincreases through nonlinear interaction with the material of the fiberand it acquires a positive frequency chirp from group velocitydispersion (GVD) in the material. The combined effect of positivenonlinearity and normal GVD serves to linearize the chirp of the pulse.The shape of the pulse remains, however, substantially unchanged (bysubstantially unchanged here we mean that the best fit to the pulseshape remains unchanged as it propagates through the fiber—in this casethe best fit is a parabolic shape.) The pulse is then amplified in thegain section 60, still retaining its substantially parabolic pulseshape. The gain segment is kept short in order to decouple amplificationof the pulse from nonlinear pulse shaping effects in this section of thecavity (pulse shaping is a hybrid of self-similar shaping andbandwidth-limited pulse shaping in the gain medium.) The amplified andpositively chirped pulse exits the gain segment of the fiber and entersthe NPE section. This section of the cavity serves as an effectivesaturable absorber. It may also serve as a free-space propagating outputport for coupling a portion of the energy of the pulse out of thecavity. The residual part of the pulse remaining in the cavity iscompressed in the net anomalous dispersion section, all the whilemaintaining its substantially parabolic pulse shape. An important aspectof this anomalous dispersion section is that there is negligiblenonlinear interaction of the pulse in this portion of the cavity inorder to avoid limitations imposed by soliton effects. Lastly,propagation through the isolator and quarter wave plate serve to preventback reflections as well as support unidirectional operation.

At this point it is instructive to compare the operationalcharacteristics of this self-similar (or “Similariton”) laser oscillatorwith those of prior art, namely the soliton and stretched pulse lasersof U.S. Pat. Nos. 4,835,778 and 5,617,434, respectively, in tabularform. TABLE 4 Type of laser Similariton Stretched-pulse SolitonNonlinear phase >>1 ˜1 <<1 Ø^(NL) Solution to the asymptotic breathingstatic wave equation Changes in pulse One minimum per Two minima perUnchanging duration per round round trip round trip trip Variation inchirp Always positive Both positive and No chirp per round trip negativeNet dispersion Normal Normal or Anomalous Anomalous

Table 4 is a table comparing the invention with prior art soliton andstretched-pulse lasers.

The preferred embodiment described herein had a total dispersion of0.005 ps² and an output pulse energy of about 2 nJ. Its spectrumexhibits the near parabolic spectral shape bounding the centerwavelength of operation shown in FIG. 5 that is characteristic of alaser oscillator. In FIG. 6 is seen the pulse width before dechirping (5ps FWHM) and after dechirping with a pair of gratings (125 fs FWHM).

In FIG. 7 is shown the spectral shape of a higher pulse energy versionof the self-similar (similariton) laser. The red curve is theexperimental results and the white curve is the result predicted by thesimulations. This particular construct had a total dispersion of 0.004ps², an output pulse energy of 6 nJ, and an output pulse width of 1.5 psFWHM before dechirping.

FIG. 8 shows an interferometric autocorrelation of the self-similarlaser of the subject invention compressed to 50 fs and possessing apulse energy of 5 nJ. The inset curve shows the background-freeautocorrelation before and after compression.

FIG. 9 schematically illustrates the main features an all-fiber versionof the subject invention in which the linear anomalous dispersionsegment provided by the gratings in FIG. 2 is by way of illustrationreplaced by a hollow microstructure fiber 300 with anomalous dispersion.Those skilled in the art will recognize that this system would alsorequire a means to favor unidirectional operation and an effectivesaturable absorber to function as a useful laser oscillator. Theseelements and other aspects of the subject invention will be obvious tothose skilled in the art.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof. It will be understood thatvariations, combinations, and modifications can be affected within thespirit and scope of the present invention.

1. A method of operating a mode-locked laser comprising the steps of:propagating the intracavity laser pulse through a first segment havingnormal group velocity dispersion; propagating the intracavity laserpulse through a second segment with normal group velocity dispersion anddoped with an element that produces gain; and propagating the laserpulse through a third segment having anomalous group velocity dispersionand negligible nonlinearity such that the laser pulse width increasesmonotonically in the normal group velocity dispersion segments anddecreases monotonically in the net anomalous dispersion segment as itpropagates around the cavity.
 2. A method of operating a mode-lockedlaser comprising the steps of: propagating the intracavity laser pulsethrough a first segment having normal group velocity dispersion;propagating the intracavity laser pulse through a second segment withnet normal group velocity dispersion and doped with an element thatproduces gain; and propagating the laser pulse through a third segmenthaving anomalous group velocity dispersion and negligible nonlinearitywherein the pulse propagating therein is characterized by a centerwavelength of operation and a spectral bandwidth having a parabolicshape in the region bounding the center wavelength of operation.
 3. Amethod of operating a pulsed laser oscillator with a cavity comprisingthe steps of: pumping a gain segment; amplifying the laser pulse in thegain segment; and propagating the laser pulse through a first segmentcharacterized by anomalous group velocity dispersion and negligiblenonlinearity; and propagating the pulse through a second segmentcharacterized by normal group velocity dispersion and nonlinearity toproduce one maximum value of the pulse width and one minimum value ofthe pulse width in one round trip of the pulse in the cavity.
 4. Amethod of operating a modelocked laser cavity comprising the steps of:propagating the laser pulse through a normal group velocity dispersionsegment; propagating the laser pulse through a gain segment foramplifying the intracavity laser pulse, the gain segment having normalgroup velocity dispersion; and propagating the pulse through a segmentcharacterized by negative group velocity dispersion and negligiblenon-linearity to produce a substantially linear sweep in frequencyacross the temporal profile of the pulse.
 5. A laser oscillator forgenerating pulses of light comprising: an optical cavity characterizedby normal dispersion segment and an anomalous dispersion segment, a gainmedium disposed within the cavity, a pump source coupled to the gainmedium the cavity, wherein a pulse propagating in the cavity accumulatesa nonlinear phase shift of greater than 1 in one round trip.
 6. A laseroscillator for generating pulses of light comprising: an optical cavitycharacterized by normal dispersion segment and an anomalous dispersionsegment, a gain medium disposed within the cavity, a pump source coupledto the gain medium the cavity, wherein the phase of the pulsepropagating in the cavity is characterized by a positive chirp.
 7. Alaser oscillator for generating pulses of light comprising: an opticalcavity characterized by normal dispersion segment and an anomalousdispersion segment, a gain medium disposed within the cavity, a pumpsource coupled to the gain medium the cavity, wherein the pulsepropagating in the cavity possess a center wavelength of operation and aspectral bandwidth characterized by a parabolic shape in the regionbounding the center wavelength of operation.
 8. A laser oscillator forgenerating pulses of light comprising: an optical cavity characterizedby normal dispersion segment and an anomalous dispersion segment, a gainmedium disposed within the cavity, a pump source coupled to the gainmedium in the cavity, wherein the pulse propagating within the cavity ischaracterized by a frequency that varies monotonically.